Licentiate thesis: Dark Stars at the Galactic Centre

by Pat Scott (McGill University)

Friday 19 Sept 2008, 10:00 → 12:00 Europe/Stockholm

Svedbergsalen

Regions such as the Galactic centre can exhibit very high densities of dark matter. This allows large amounts of dark matter to be gravitationally captured by stars resident in these regions. I show that if the dark matter consists of weakly-interacting massive particles (WIMPs), the energy released by self-annihilation of captured WIMPs can significantly alter the structure and evolution of main-sequence stars, with low-mass stars the most affected. I describe the 'dark' stellar evolution code DarkStars, and compare the results obtained with it to those from a dark version of a simple static stellar structure code. I then present a series of detailed grids of WIMP-influenced stellar models for main sequence stars, describing the changes in stellar structure and main sequence evolution which occur as a function of the rate of energy injection by WIMPs, for stars of 0.3--2.0 solar masses and metallicities Z = 0.0003--0.02. I illustrate the rates of energy injection which can be expected using realistic orbital parameters for stars at the Galactic centre, including detailed consideration of the velocity and density profiles of dark matter. Capture and annihilation rates are strongly boosted when stars follow elliptical rather than circular orbits. Dark stars are unlikely to exist on any circular orbits in the Milky Way. If there is a spike of dark matter induced by the supermassive black hole at the Galactic centre, solar-mass stars following orbits with periods as long as 50 years and eccentricities as low as 0.9 could be significantly affected. Any observation of low-mass stars on such orbits would provide limits upon WIMP properties of comparable sensitivity to current direct-detection experiments.